The invention relates to communications systems which transfer data between a plurality of nodes all of which are connected to a common conducting cable, particularly a cable that differentially conducts data signals.
There is an increasing use of local area network systems for the transfer of information between a plurality of information processing sites. A number of standards have been promulgated by the industry which characterize and define structure and operation of such systems. One such standard is the Ethernet system which ties together a plurality of information nodes on a common databus. An Ethernet node includes a cable-transceiver interface which taps the Ethernet bus and communicates with data terminal terminal equipment through an Ethernet drop cable. Signals are transmitted differentially on the Ethernet drop cable and an Ethernet information node must include a cable driver capable of differentially driving the cable according to the Ethernet specification.
One mode of signal transmission on the Ethernet drop cable involves the use of inductive coupling to place a differential signal on the cable. In this mode, when a transmitter is idle, that is when the node is not transmitting, the transmitter output must provide a zero differential in order to minimize the total offset voltage present on the cable. Another specification requires that the waveform provided by a transmitter when it ends an active transmission and transitions to its idle state remain at one level for a minimum period of time and then transition to the zero differential level.
One integrated circuit family that is easily adapted to the differential transmission requirements of the Ethernet standard is the ECL family. An ECL driver characteristically responds to an input which can be driven differentially to produce a differential output. One problem with using an ECL driver to interface a node with an Ethernet transmission cable is that its open loop voltage gain is high, typically in the range of 20-30, which tends to amplify input and internal parametric offset voltages.
Further, the conventional ECL circuitry involved in meeting the active-to-idle transition requirement changes the output voltage abruptly, resulting in a common mode step change in the differential outputs which is propagated and reflected in the Ethernet cable.
The present invention is used to advantage as an Ethernet driver. It employs a minimum of internal circuitry to perform the dual function of minimizing the output offset voltage during the idle state and meeting the transition requirement without introducing an output common mode step change into the cable.